Fender Champ 5F1: lowering the B+ 230/240V + biasing

This nicely built Fender Champ clone was dissipating 19W while the output 6V6GT is only rated for 14W. Although or exactly because it was built strictly to the Fender 1957 schematic, the voltages do not check out. The power transformer that was supposed to give 330V, is delivering +400V. European builds seem to suppose that wall voltage is 230V while after measuring, I found it to be 238+V. Connecting to the 240V winding brought the voltage down even more. Further biasing was done by changing the cathode resistance.

[@FraJa1980]

Holy crap! 19 watts from a 6V6 is insane! Good job on taming it tho, sounds very good when it has a little headroom to toy with.. I've been watching a lot of your videos lately and I must say something that is at least as impressive as your electrical skills is how easy you can talk to the camera! I tried that a few times and my English is good but I always end up saying "Uhm.." every 20 seconds because it's weird talking to someone who isn't there. I can respect anyone with good articulate skills and know how to explain stuff for videos like this because that's what makes the video worth watching.

[@Bigjoedo66]

I just had the same issue with a champ build. I changed the 470Ω bias resistor to 1KΩ 5 watt resistor. Unfortunately, I found your video after I made the changes. Great video by the way!!

[@ianaintsaying1625]

I've built several 5F1 Champs using a 600Ω cathode resistor instead of the schematic's 470Ω. This lowered the plate dissipation perfectly.

[@zjokka]

Thanks so much for chiming in -- seemed like sacrilege at the time, but really: the context is different: wall voltages and transformers are not what they were in the 50s!

[@jimcatanzaro7808]

I use a single 6l6 in my champ I built 420v b+ to the output transformer with a 50ma choke to 350v to the plate and 225 to the 12ax7 I use a pot for the bias since I like to swap tubes often

[@tomk1tl39]

I built a 5F2 and used 1KΩ / 3W cathode resistor . . . works OK so far as the 470 Ω was running too hot and tubes are not cheap anymore . . .

[@oldasrocks9121]

95% of plate dissapation for sIngle ended amp according to the disappation value for plate voltage listed in the TUBE MANUAL. That would be 11.4 watts for a 6v6GT. The tube should then last for its rated duty cycle of 4-5000 hours. Again, the TUBE MANUAL-- You alter tone with the plate and grid leak resistors on the preamp triode, the power tube bias will change tone a little but not as much as changing preamp values. Increase those resistance values. You can also add a bypass cap to the driver stage cathode for more gain, try 2.2uf or 4.7 uf. Stabilize unruly uncontrolled distortion with negative feedback, temporarily install a B100K pot wired as a rheostat between the positive speaker output and the driver cathode. If you've installed a bypass cap you can install it at the ground end of the bias resistor and bypass cap with a 47 ohm tail resistor to ground, see the AA764 (maybe AB764?) schematic. Adjust the pot to find a fixed resistor value or install the pot if the owner doesn't mind a new hole in his chassis. 100K pot is enough to effectively turn the NFB off turned fully clockwise.

[@efenderbls]

A 6V6GT is happy at 12w safe max and a JJ 6V6S can handle 14w I when I first built my 5F1 Champ I didn't use a cathode bypass capacitor on V1. As some call it running a tube without is cathode degeneration or cathode constipation haha but it really does help a lot. I originally tried a 25uF but was getting some unwanted distortion blocking so I then installed a 4.7uF and that cleared up most of the distortion blocking. I then lowered the two .022uF coupling caps to a .0074uF and now she cools with smooth raw distortion without the raspy distortion blocking. Thar was very cool to hear the difference when you put the V1 cathode bypass capacitor in and out of the circuit you can really hear a big change.

[@whatsstefon]

Looking forward to the finale. Are you planning on bringing everything to spec? Or will you leave the higher than spec plate voltage, and just bias it to approximately 14W dissipation?

[@zjokka]

Hi Stefano, thanks so much for your question! I haven't gotten to the bottom of this. I would really like to see some real time measurements of a vintage 1957 Fender Champ plugged into a modern US wall socket. It might just idle at 400V too... If you check D-Lab video on a Blackface Champ, you see it's got about the same B+, about 400V.... Probably the power transformers in those OLD Champ were different from the later ones. Changing the power transformer connection to the 240Vac, brought it down from 425V, changing the bias resistor to about 750R --original has 470R-- did really help the constipated, compressed sound of this amp. It's now biased at a little under 14W. It's not my amp, and as you can tell it is built by a bunch of fetishists who make sure the install the useless RCA speaker connection, but don't check the bias of the amp. You could use zener diodes to bring the B+ down even further. If it were my amp, I would certainly have tried it. I am not sure whether lower voltages would improve the sound and as modern tubes are more sturdy and can certainly handle 400V.

[@whatsstefon]

zjokka yeah, if the B+ is within spec of the tubes, then bias to get the 14W dissipation. D-labs mentioned the issue once that these old amps ran on 110VAC. That 10VAC difference on the primary makes a difference in the secondaries. Perhaps the transformer isn't wound accurately? If this is a Chinese supplied build, maybe and usually the specs of their components comes with significantly wide tolerances. I was thinking of purchasing one of these kits excluding the transformers, but the reality is, the resistors, capacitors and wiring are just not up to spec. So you're better off purchasing high quality transformers from a reputable source. And maybe just buy the kit parts separately, such as the chassis if you're not a good metal worker, the cabinet if you're not a good wood worker.

[@zjokka]

It's a complex issue, they really do have 240V split in two... then with resistive loading in the house wiring that 120V may water down to 117V. What you really get also depends on the electrical company. I don't think you will ever see 123V. Here I did see 238V -- and every electrical person agrees I should connect the 240V.... Australia has 230V too right. What's actually in the wall? www.worldstandards.eu/electricity/plug-voltage-by-country/ Transformer winding: yeah good point... made me think of this: a) US: we need to bring 110V to 330V, that's 1:3 b) rest: wee need to bring 240V to 330 V that's 1:1.35 Inaccuracy in winding the second transformer will have a bigger influence on the result! Yes transformers are the big problem: if not the quality, then the price. That's why I will never again buy a kit -- for the price of the transformers I can easily buy an old tube PA amp with transformers and chokes the whole shabang. Some of them I paid less than €100 for a 18W-marshall-to-be... 2x 12AX7; 2xEL84; EZ81 recitifier.

[@Blueguitar007]

You're plugged into input 2 - attenuated.

[@davydmir6565]

Hi i'm wondering if you could help me? You see Im trying to figure out if I can make a 5f1 run on uk 240v? I can't find any info about this online, do you have any resources or tips? thanks in advance!

[@zjokka]

kzbin.info/www/bejne/fmqkhGqCo8iYe5I this video shows how I run my 120V US power transformer loaded Danelectro here in Europe, with an extra transformer that only cost me 50 EUR. It allows you to keep the original transformer, which is a huge part of the sound with hand wound vintage power transformers. Otherwise, changing the power transformer is the only way to go. Transformers are usually either converting 120V or 240V to the voltages needed in the amp. There are one that have 120V and 240V taps on the primary, so do check. Hope this helps, can't say much more without knowing the amp.

[@mikecamps7226]

The issue is that you need to look at a TUBE DATA BOOK ! You want to run the 6V6 under 400 volts for the plates. You noticed it on the old schematic but didn't understand. So you need to insert an extra filtering stage off the 5Y3 rectifier with a dropping resistor and cap....before the power rail hits the output transformer and the power tube plate connection node.....so that node gets down to approximately 350 volts to 380 volts. The higher the plate volts go, the more stiff the power tube will sound....harsh and sterile and bright without good rich tone. Once you get the plate voltages within that target range...then you can bias the power tube with the proper value cathode resistor......and calculate the plate dissipation factor for class A operation.

[@mikecamps7226]

You have to understand that this is a cathode bias amp with a rectifier tube, and not a fixed bias situation. The cathode bias has what is called SAG where the voltage will drop due to the load demand on the power supply and tube....to amplify the signal its being presented. The voltage will SAG down and then try to catch up to where it was supposed to be set......so the tube can actually be over biased hot......as long as the plates do not red plate. The SAG is the interaction of the rectifier tube and the cathode bias set up.

[@zjokka]

Hey dude, I will get back to this amp one time, but it is not mine. Somebody below in the comments spotted a wrong value in the power rail -hehe. It's a nice little amp but the guy who owns it never liked it much (harp player) and that also what I sensed: stiff and overbiased... Thanks for replying I learned something... only because I had the balls to post my mistakes to the world... but Mike C Amps - your channel has no content. I'd subscribe. Thanks again for your pointers!

[@mikecamps7226]

@@zjokka The power rail resistors in the schematic should be a 10K and a 22K but its the voltage feed in to that first 10K node which will vary till the cathode bias is set right. Then if its still up over 350 volts....is when you determine adding another node between the rectifier and that first 10K node and how big of a power resistor you will need to get to the voltage goal.....but again doing so will also need to determine the cathode bias all over again to zero in to the right values but you'll know which way to go by then. The power transformers they supply are actually existing transformers for other amp platforms so they weren't specifically made for the tweed amp in this application and generally at a slightly higher voltage than what would have been used in the 1950's in applications.

[@mikecamps7226]

@@zjokka Here is the tip for a harp amp application.......if the guy is using a microphone that has a volume control.....change the pre-amp tube from the 12ax7 to a 12au7 and the cathodes of that pre-amp tube should by UNbypassed.....no bypass cap. The gain and signal output from a harp microphone extremely exceeds what an electric guitar will generate to drive the amplifier. So a 12au7 has the least amount of gain in that tube family. So its either the 12au7 or a 12ay7....the AY is the next gain step up. If the 12au7 is still too much......then you need to increase the cathode resistor value on the first stage of that tube that gets fed by the input jack. Its a 1.5K and you can go up as that will soften the tube and bring down its gain......so you have to adjust with playing the amp with the microphone to dial it in. You might also have to soften the gain in the opposite side too but start with the primary side. The whole thing will be about the microphone volume control sweep and how it hits the amp and as the harp player blows and draws and the energy they put into those mechanics for the dynamics. The ideal target is to have the amp as loud as it can get....and the distance from the amp with the mic.....to avoid the screech of feedback. the harp guy will have less feedback facing away to play as opposed to facing direct to the amp...all when cupping the harp mic......but they ride the mic volume control playing harp since its right there at their fingers

[@mikecamps7226]

once you get the amp dialed in that way......try removing the feedback resistor going from the output transformer to the pre-amp tube

[@RankoTomic]

So what's the conclusion? Which bias fits this the best? How did you lower B+?

[@zjokka]

Hi Ranko, thanks for the question. Switching the primary to 240v did make a difference, but not a big one. Looking at the original Champ schematics from Fender, they biased their all their Champs very very hot. B+ is not really a problem, you regulate bias by using a bigger cathode resistors than Fender did. This amp is not mine...but wouldn't mind having another look at it. Out of my hands for now.

[@gobigrey9352]

I just bought a new 5f1 from Lil' Dawg. It sounds as clean as this one. Really only breaks up at max volume. I'm wondering if there's something wrong with it.

[@zjokka]

Gobi Grey thanks for your question. Break up really depends on the bias of output tube. But Fender never really intended this breakup and was trying to create clean amps. If you are not able to check bias and change resistors in the amps, you could try a lower gain tube like Sovteks. Did you contact Lil Dawg with this issue?

[@gobigrey9352]

Ok, thanks, I'll contact him.

[@zjokka]

The clip on his website might wel have been recorded with the thing cranked. Did you check the end of my video, where it start breaking up a little earlier? Breakup will always be around 3/4th of the way up. Let me know how it turns out!

[@gobigrey9352]

I feel like an idiot. My delay pedal was in the loop position which lowers the volume even when it's off. Amp works fine and sounds great!

[@zjokka]

Everybody feels like that from time to time, only real idiots never learn and never dare to admit their mistakes. Thanks for the feedback! Enjoy the amp!

[@Fulou]

Silly question from the inexperienced, why didn't you just move the OT from B1 to B2 as that DC was right on for voltage? At least on the previous video when you measured it?

[@zjokka]

Hi Fulou, I did change it, but under load (when connected) the differences were minimal. Thanks for the question, none too sily. Sorry for the late reply...

[@Fulou]

@@zjokka thanks! It's a big help :)

[@jimcatanzaro7808]

Your Filliment ac is probably too high causing it to burn out

[@zjokka]

Great comment. One of the most critical voltages in any amp -- I'll get back to this one soon!